1. Technical Field
The present invention relates to a torque detecting device for detecting rotational torque applied to a rotary shaft, and a manufacturing method of a yoke assembly for use in the torque detecting device.
2. Description of Related Art
An electric power steering apparatus which assists steering by driving a steering assist motor according to the rotating operation of a steering member, such as a steering wheel, and applying the torque of the motor to a steering mechanism comprises a torque detecting device for detecting the steering torque applied to the steering member for use in the drive control of the steering assist motor. In this torque detecting device, a steering shaft (rotary shaft) connecting the steering member and the steering mechanism is divided into a first shaft and a second shaft which are connected together coaxially with a small-diameter torsion bar, and the steering torque (rotational torque) applied to the steering shaft by the operation of the steering member is detected using a relative angular displacement caused between the first and second shafts with the torsion of the torsion bar.
Conventionally, the relative angular displacement between the first and second shafts is detected by various means. One of the means is a torque detecting device including a cylindrical magnet which rotates together with the first shaft, and one set of two yoke rings which surround the outside of the cylindrical magnet and rotates together with the second shaft, and constructed to detect the relative angular displacement by using a change in a magnetic circuit formed between the yoke rings and the cylindrical magnet (see, for example, Japanese Patent Application Laid-Open No. 2003-149062).
The cylindrical magnet is constructed as a multi-polar magnet including a plurality of N poles and S poles arranged alternately by integrally retaining a plurality of rectangular magnet pieces in a retaining cylinder made of a resin molded in the shape of a cylinder, and externally fitted and fixed on the first shaft via the retaining cylinder. The two yoke rings are thin rings made of a soft magnetic material. On the inner circumference of each yoke ring, the same number of pole claws as the number of the magnetic poles of the cylindrical magnet are arranged equally in a circumferential direction and extended toward one side in an axial direction. These yoke rings are placed so that the extending sides of the respective pole claws face each other and that the respective pole claws are arranged alternately at equal intervals in a circumferential direction. The yoke rings are integrated while keeping this state with a retaining cylinder made of a resin molded in the shape of a cylinder to form a yoke assembly, and the yoke assembly is externally fitted and fixed on an end of the second shaft on the connected side via a metal collar integrally molded into one side of the retaining cylinder.
The cylindrical magnet and the yoke assembly thus fixed are mounted so that the pole claws of the two yoke rings aligned and exposed at the inner circumferential surface of the retaining cylinder are aligned respectively with the boundaries of N poles and S poles on the external circumferential surface of the cylindrical magnet. Therefore, when a relative angular displacement is caused between the first and second shafts by the application of steering torque to the steering shaft, the positional relationships in the circumferential direction between the pole claws of the two yoke rings and the magnetic poles of the cylindrical magnet change mutually in opposite directions. Since the magnetic flux generated in the two yoke rings changes according to this positional change, it is possible to find the steering torque by detecting the change in the magnetic flux.